A Brief Guide to Petrol Octane Ratings: RON, MON, and AKI

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Whenever we Malaysian drivers stand in front of a petrol pump, we see quite a few options in front of us. Regardless of which petrol brand we use, each one offers selections that are differentiated with abbreviations and numbers such as “RON 95” or “RON 97”.

We may be surprised, however, when we visit other countries and see that their petrol pumps offer different options with labels like “MON” or “AKI”. These many different labels have also resulted in some misconceptions about the fuel we use; such as the assumption that the more expensive fuel is better for our engines, resulting in more horsepower and higher speeds (that’s not necessarily true, by the way!).

But do we really know what all of these terms mean? The truth is that not all fuels are equal, and we should be using the right fuels that are rated for the engine we have in our car. One way to know what fuels are optimal for our cars are those labels we just mentioned: RON, MON, and AKI, all of which are known as Octane Ratings for fuel.

What They Mean

Octane Ratings are important because they indicate to us how those fuels actually behave as they flow through our engines, get mixed and compressed with air before combusting and generating power. If we were to use a fuel with an octane rating too low for our engines, this may result in ‘knocking’ or ‘pinging’ caused by an air/fuel mixture that’s not right for our engine causing the fuel to combust too early.

The good news is that this problem is easy to avoid: referring to our car manual, we can see exactly the type of fuel we need for our cars. As long as we use fuel with that octane rating or above, there won’t be a problem. It’s only a problem if we use an octane rating that’s too low, or if we put diesel in an engine that requires petrol!

Fuels with Low Octane Ratings

What happens if we use fuel that has too low of an octane rating? Well, this could cause our engine to run poorly and affect overall fuel and power efficiency. Long-term, this could damage our engines and the car’s emission control system, and it could also cause loud knocking and pinging sounds to come out from the engine. Depending on your car manufacturer, using the wrong fuel may even void your warranty!

Higher-Octane Fuels

So what cars actually need those pricier fuels with higher octane ratings? They are cars with engines that need higher compression for fuels or the ones that use super/turbocharging to force more air into the engines are the ones that require higher-octane fuels which won’t ignite prematurely. When these kinds of engines use fuels with higher octane ratings, they’ll be able to maximize energy output and also fuel efficiency.

The three types of octane ratings

There are three commonly-used methods for producing Octane Ratings. These are the Research Octane Number (RON), the Motor Octane Number (MON), and the Anti-Knock Index (AKI). While there are quite a few differences between these three, the basic difference is in how they calculate their ratings.

Research Octane Number (RON)

RON, which is the rating we typically see in Malaysian petrol stations and in many other parts of the world, measures how the fuel behaves at lower engine temperatures and at lower speeds. This measurement is achieved by testing the fuel in test engines at 600 rpm. Under those controlled conditions, the fuel is subjected to many different levels of compression.

Motor Octane Number (MON)

MON, on the other hand, is tested in engines at 900 rpm. The fuel mixture is pre-heated and the ignition timing (the timing of when the fuel is ignited) is also varied.

For both RON and MON, the objective of testing the fuels while changing the many different variables is to test how the fuels are able to resist knocking, or how much it can take before it causes those knocking or pinging sounds mentioned earlier.

Anti-Knock Index (AKI)

Slightly less common is the AKI. This is easier to calculate as it is simply the average of the RON and MON ratings of the fuel, which is why it’s also referred to as ‘(R+M)/2’. Other names for this rating also include the Posted Octane Number or PON and is most commonly seen in the United States, Canada, and Brazil.

What It Means For The Average Driver

So what does this all mean for the average driver? Well, firstly we should check our car manual to know for sure what fuel is most appropriate for our engines. Second, we should not believe the misconception that using a higher-rated fuel will give us better performance; it won’t damage our engines, but it won’t give us better performance either, even though we’ve paid more for it. Lastly, if we were to ever drive in another country, perhaps if we were to rent a car while on vacation overseas, we need to be aware that the octane ratings we see when fueling our car may not be the same as those in Malaysia.